Many knives or shear bars used in machines designed for agricultural applications such as crop harvesting and animal feed processing have a hardfacing component to protect their cutting edges. The knives or shear bars are hardfaced for a number of reasons such as to provide wear protection, to limit the amount of blunting during service and to extend the service life of the cutting edge.
Agricultural machine knives or shear bars are traditionally hardfaced by thermal spraying of, for example, a nickel based metal powder, which forms a binder matrix containing hard particles such as tungsten carbides, followed by a fusing (sintering) process.
There are limitations to thermal spraying in that it generally uses pre-blended powder alloys with a limited amount of hard particle content. Typically, this limit can be in the region of 50% hard particle content. This is because the spray process requires the binder matrix to retain the hard particles within the hardfacing and, if the percentage of hard particle content gets too large, the hard particle will not be retained on the target substrate.
The agricultural machine industry has overcome this limitation by using build up welding technologies such as Plasma Transferred Arc (PTA) Welding; expensive single stage spraying technologies such as High Velocity Oxy-Fuel (HVOF); or brazing technologies.
PTA, as well as some other processes, generate a large amount of heat, which affects the substrate. So, in the case of sharp knives, PTA would damage a sharp or near sharp machined cutting edge.
Laser surfacing technology also referred to as laser cladding, laser coating, Direct Laser Deposition or laser powder deposition is a process that involves the deposition of hardfacing on a substrate, using a laser heat source to fuse a thin layer of metal to the substrate.
In general, there are two basic techniques for laser cladding. The first is a two-step process where hardfacing material is deposited on a substrate followed by a fusing pass with a laser heat source. The second is a one-stage deposit pass where the additive hardfacing material is deposited into a moving melt pool by a low pressure feed. Laser cladding enables a higher ratio of hard particle to binder alloy in the composition. Nonetheless, laser cladding process can lead to stress cracking in the deposited hardfacing, which can result in a less effective knife or shear bar.